NEW YORK (Reuters) - Days after the killing of Osama bin Laden, U.S. President Barack Obama met New York firefighters and police on Thursday and visited Ground Zero to offer comfort to a city still scarred by the Sept. 11 attacks.

His predecessor, George W. Bush, just three days after hijacked planes destroyed the World Trade Center's Twin Towers, had stood bullhorn in hand in the smoldering wreckage to declare, "The people who knocked these buildings down will hear all of us soon."

Almost a decade later, in a bookend to that historic visit, Obama came to New York to say that promise had been kept.

He said the killing of bin Laden told the world "that when we say we will never forget, we mean what we say."

Obama visited Engine 54 in midtown, which with 15 deaths lost more members on 9/11 than any other firehouse, before heading to Lower Manhattan to talk with police and lay a wreath at Ground Zero, the Twin Towers site, where he also met with victims' families.

Obama told firefighters at the "Pride of Manhattan" firehouse, "I wanted to just come here to thank you."

"This is a symbolic site of the extraordinary sacrifice that was made on that terrible day almost 10 years ago," he said. "It didn't matter who was in charge, we were going to make sure that the perpetrators of that horrible act -- that they received justice.

Talat Hamdani, 59, whose New York police cadet son, Salman, 23, was killed in the Sept. 11 attacks, met Obama along with other families of victims at the World Trade Center site and said it was a "very healing" experience.

"I thanked him for being there for me today and ... that I was very proud of him as our president," said Hamdani, who moved to the United States from Pakistan. "He was there sharing our feelings ... many people broke down."

She cried as she showed a picture of Salman and told Obama her Pakistani-born son had been a "proud American."

Bin Laden, the al Qaeda leader who masterminded the Sept. 11, 2001, attacks, was shot in the head by U.S. forces who stormed his compound in Pakistan on Monday after a decade-long manhunt. Nearly 3,000 people were killed when al Qaeda hijackers crashed commercial planes into the Twin Towers, the Pentagon outside Washington, and a Pennsylvania field.

"We have been waiting for this for 10 years. It puts a little more American pride in people," said Al Fiammetta, 57, a safety engineer from Bellport, New York, who said he worked at Ground Zero clearing debris and waited to see Obama.

New York City resident Caroline Epner, 32 said, "It's OK for him to take a victory lap."

RED, WHITE AND BLUE

Obama later met New York police, thanked them and urged them to be vigilant, saying extremist threats remained.

At Ground Zero during a sunny afternoon, Obama laid a wreath of red, white and blue flowers to honor those who died. He then paused for a moment of silence.

Obama, who made no remarks at the site, greeted relatives of victims. The brief ceremony took place by the "Survivor's Tree," which survived the attacks and was nursed back to health and then returned to be part of the memorial that will open on the 10th anniversary of the attacks.

He stood in a place that almost a decade ago was the pulverized remains of what were once the world's tallest buildings, which for weeks after the attacks spread a ghoulish dust over Lower Manhattan.

Visible progress in the $11 billion project to rebuild the World Trade Center site is now being made after delays from political, security and financing concerns. The 1,776-foot (541-metre) centerpiece, 1 World Trade Center, already stands more than 60 stories high.

Democrat Obama had invited Bush to join him, but the Republican declined, saying through his spokesman he had preferred to remain out of the spotlight since leaving office in 2009.

Some among the thousands at Ground Zero, where many waved American flags, said they would have liked to see Bush return to the site on Thursday.

"I want to thank Bush for what he started and Obama for what he finished," said Al Smith, 52, who said he delivered newspapers to the Twin Towers hours before they collapsed.

At the Pentagon, Vice President Joe Biden placed a wreath by a blackened stone, charred by flames after a plane crashed into the building, inscribed with the words "September 11, 2001" which honors the people killed there.

September 11th Families for Peaceful Tomorrows urged Obama to close the U.S. military prison housing foreign terrorism suspects at Guantanamo Bay in Cuba and bring home American troops in Afghanistan and Iraq.

Bin Laden's killing coincided with the first anniversary of a failed attempt to bomb New York's Times Square, one of at least 11 plots against the city since 9/11.

Several recent polls showed Obama's job approval rating boosted after bin Laden's death, although such bounces are often short-lived. Obama's popularity before the 2012 election where he is seeking a second term had been hurt by economic woes and high gasoline prices.

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